The shortage of radioactive isotopes due to the temporary closure of an Ontario nuclear reactor is leading patients scheduled for diagnostic tests to make other plans.

Thousands of patients in Canada, the United States and other countries have had their medical tests postponed because of the shutdown of the government-run Chalk River reactor. It had initially been scheduled for only one week for maintenance repairs, but now could be shut down until mid-January.

The reactor is the source of more than two-thirds of the world's demand for medical radioactive isotopes, which are used to diagnose cancer and cardiovascular diseases. It yields enough isotopes to treat more than 76,000 people a day — more than 20 million a year.

Les Blatchford, a cardiac patient from Beaconsfield, Que., told CBC News on Thursday that he was scheduled to have a heart test this past Tuesday. The test, which involved the injection of a radioactive dye into his bloodstream to assess blockages in his arteries, was cancelled until Dec. 18 because of the shortage.

"They were very decent about it," he said of the hospital that cancelled the appointment.

But Blatchford wants to know why the medical industry is so dependent on Chalk River for the source of this material. "Why wasn't an additional supply standing by?" he said.

On his doctor's recommendation, Blatchford called a private medical clinic in his area, which, despite the critical shortage, has agreed to perform the procedure Friday.

Blatchford said clinic staff assured him they had technetium-99, an isotope of technetium. But he said other Canadians may not be as fortunate. "I'm concerned that there are people who are in a much worse situation than I am," he said.

Once injected into patients, the radioactive isotopes allow radiologists to zero in on areas of higher radiation and to pinpoint key changes in the body to make accurate diagnoses.

Ottawa-based MDS Nordion distributes the isotopes, which can't be stockpiled because they have a short shelf life.